A party is always better if it has a specific theme. Of course, this is every bit as true for patio parties too. Often a patio party will revolve around a certain food using other complimenting foods and décor as part of the theme. For this reason, Italian food, Mexican food, French food, Chinese food, all work well for the basis of a theme.
Of course, you could use any fare that you like. A group of young people would probably enjoy a party centered on a pizza theme. Set some tables with checkered tablecloths, place some old wine bottles holding lit candles on some safe tables set back from the action, and be inspired with any other decorations that might remind you of an old pizza parlor.
Serve pizza with soft drinks, along with snack trays of olives, vegetable sticks, tomato wedges, and pepperoni slices or salami slices. For dessert, consider Italian pastries or any other Italian delicacy you enjoy. Italian ices would go over great in hot weather.
Something unexpected would bring enjoyment to your guests. For an adult party, you can also use a particular wine, group of wines, or drink as the theme. The decorating for the patio party can continue the theme that you choose, so that if it is a Mexican theme, go to a store that carries items styled in a Mexican style and decorate the patio to match the theme. Put on some music with a Salsa flavor and the party has begun.
Theme ideas for patio or deck parties are practically unlimited. However, narrowing your party's focus to one specific niche will guarantee a memorable party for all! Most importantly, be sure to enjoy your patio!